1. Identify the situation

   This is the case of Newton’s rings in reflected light.

   A thin air film is formed between:

   • the lower convex surface of the plano-convex lens, and
   • the upper plane surface of the glass plate.

   The two interfering reflected rays come from:

   • reflection at the lens-air interface (top surface of the air film),
   • reflection at the air-glass plate interface (bottom surface of the air film).

2. Check Statement-1

   Statement-1 says:

   When light reflects from the air-glass plate interface, the reflected wave suffers a phase change of $\pi$.

   Rule of reflection:

   • Reflection from a rarer to denser medium gives a phase shift of $\pi$.
   • Reflection from a denser to rarer medium gives no phase shift.

   Here, reflection is at the air-glass plate interface:

   • air is optically rarer,
   • glass is optically denser.

   Therefore, the reflected ray indeed suffers a phase change of $\pi$.

   So, Statement-1 is true.

3. Check Statement-2

   Statement-2 says:

   The center of the interference pattern is dark.

   At the center of Newton’s rings, the film thickness is $t=0.$

   The geometrical path difference between the two reflected rays is then zero, but one of the rays gets an additional phase shift of $\pi$ due to reflection at the air-glass interface.

   Hence the effective path difference is $\frac{\lambda}{2}$ in phase terms, producing destructive interference.

   Therefore, the center is dark.

   So, Statement-2 is true.

4. Check the relation between the statements

   • Statement-1 explains that one reflected ray gets a phase change of $\pi$.
   • Because of this phase reversal, even when $t=0$ at the center, the two reflected rays interfere destructively.
   • Hence the center is dark.

   Therefore, Statement-1 is the explanation of Statement-2, not the other way around.

   But option A says Statement-2 explains Statement-1, which is incorrect. Option B says both are true, but Statement-2 is not the correct explanation of Statement-1. This is correct.

5. Evaluate options

   • A: Both true, but wrong explanation direction $\Rightarrow$ Incorrect
   • B: Both true, and Statement-2 is not the correct explanation of Statement-1 $\Rightarrow$ Correct
   • C: Statement-1 false, Statement-2 true $\Rightarrow$ Incorrect
   • D: Statement-1 true, Statement-2 false $\Rightarrow$ Incorrect

Therefore, the correct option is B.